High-intensity (or high-optical-power) light beams are used in many applications, including thermal processing of semiconductor substrates. Most applications require the high-intensity light beam to have a well-defined intensity profile. For example, in laser annealing applications, the high-intensity beam forms a line image and has a generally Gaussian intensity distribution along the short axis and a generally uniform intensity distribution along the long axis. Typical dimensions for the line image used in thermal processing of semiconductor substrates are hundreds of microns in width (short axis) by a few tens of millimeters in length (long axis). The amount of power in such line images can reach a few kilowatts.
It is difficult to accurately measure intensity characteristics, such as an intensity profile, of such high-intensity light beams because the light beam damages the measurement apparatus. One type of measurement apparatus uses image sensors and an attenuator to attenuate the high-intensity beam down to a reasonable (non-damaging) power level and then directs the attenuated light beam to a photodetector such as a CCD or CMOS camera.
Unfortunately, this approach suffers from significant inaccuracies because attenuation is always accompanied by aberrations and because measurements at low power do not accurately represent the intensity profile distributions realized at the high power at which the light beam is actually used.
Another type of measurement approach is based on scanning the high-intensity light beam past a narrow aperture (e.g., a slit aperture) formed by opposing blades. When high-power densities are involved, however, the measurement needs to be carried out at low-power settings. This reduces the measurement accuracy for essentially the same reason as the image-sensor-based measurement methods. On the other hand, trying to measure the intensity profile at high power to get an accurate measurement typically results in overheating and thereby damaging the blades. Thermal expansion of the blade material can also change the size and/or shape of the slit opening and compromises the measurement. This can occur even at low power.
Measurements of the short-axis intensity profile of a line-forming light beam are even more challenging than the long-axis measurements because scanning in the short axis direction requires a very small slit or even a pinhole. Thermal expansion of a small aperture is more pronounced than thermal expansion of a large aperture. For this reason, measurements of the intensity profile along the short axis of a line-forming light beam are usually done using cameras with a large degree of attenuation or even below the laser threshold. But, as noted above, the measurement accuracy suffers.